1. The Field of the Invention
The present invention relates generally to optical transmitters. More specifically, the present invention relates to mechanisms for controlling the extinction ratio of an optical signal by modulating the optical signal levels using a frequency spread tone.
2. Background and Relevant Art
Computing and networking technology have transformed our world. As the amount of information communicated over networks has increased, high speed transmission has become ever more critical. Many high speed data transmission networks rely on optical transceivers and similar devices for facilitating transmission and reception of digital data embodied in the form of optical signals over optical fibers. Optical networks are thus found in a wide variety of high speed applications ranging from as modest as a small Local Area Network (LAN) to as grandiose as the backbone of the Internet.
Typically, data transmission in such networks is implemented by way of an optical transmitter (also referred to as an electro-optic transducer), such as a laser or Light Emitting Diode (LED). The electro-optic transducer emits light when current is passed through it, the intensity of the emitted light being a function of the current magnitude. Data reception is generally implemented by way of an optical receiver (also referred to as an optoelectronic transducer), an example of which is a photodiode. The optoelectronic transducer receives light and generates a current, the magnitude of the generated current being a function of the intensity of the received light.
Information is conveyed over an optical fiber by transmitting different optical intensities on the fiber. A relatively high optical power level is transmitted onto the optical fiber to assert one binary value onto the fiber. A relatively low optical power level is transmitted onto the optical fiber to assert the opposite binary value. There is also an average optical power somewhere between the high optical power and the low optical power. The high optical power is obtained by asserting a higher current to the laser. The low optical power is obtained by asserting a lower current to the laser. The laser is not turned off because it takes significant time to saturate a laser to the point where it begins to lase if starting from a laser that is off. In fact, if the current through the laser were to drop below a certain threshold current, it can take much longer to transition to the high optical intensity. In high data rate applications, this could cause significant jitter and possible degradation of the signal. Accordingly, even the low current that enables the low optical intensity should be kept above the threshold current of the laser. If this constraint is met, the laser can transition quickly from the high optical level to the low optical level, and vice versa.
An additional constraint to the high and low optical levels is referred to as the “extinction ratio”. The extinction ratio is the ratio of the high optical power level to the low optical power level. The optical high and low power levels are obtained by modulating the current between the higher and lower currents. Typical extinction ratio values range from perhaps 6 dB to 12 dB, with higher levels generally being better but more costly in terms of power requirements.
However, maintaining of a proper extinction ratio is more complex than simply statically determining an appropriate high optical level and an appropriate low optical level, and keeping with that level. Varying temperatures have a profound effect upon the extinction ratio. FIG. 4 illustrates approximate laser current versus optical power curves for several different temperatures including 0, 25 and 70 degrees Celsius. The threshold current for 0, 25 and 70 degrees Celsius are illustrated as ITH0, ITH25 and ITH70. The difference in the curves for varying temperatures is exaggerated to illustrate the principles of temperature dependency in the curve. Each laser will have slightly different curves shapes and temperatures dependencies. However, regardless of the laser type and make, the laser tends not to emit significant optical power if the supplied current is below the threshold current. In addition, for all lasers, as temperature rises, threshold current increases and the slope of the curve in the linear region above the threshold current (i.e., the slope efficiency) reduces.
FIG. 4 also shows the low optical level PLOW25 for 25 degrees Celsius and the corresponding current ILOW25 needed to attain that low power level at 25 degrees Celsius, and a high optical level PHIGH25 also for 25 degrees Celsius and the corresponding current IHIGH25 needed to attain that high power level at 25 degrees Celsius.
As temperature rises, the threshold current needed for the laser to transmit any significant degree of optical power rises. In addition, the slope of the curve in the linear region above the saturation current becomes less steep. This means that if the temperature were to fall or rise, the optical power emitted by the laser given a constant current will also change. Accordingly, in order to maintain a proper extinction ratio, the extinction ratio is periodically checked and adjusted if needed. This allows the optical transmitter or transceiver to operate under wide-ranging temperature conditions without introducing inordinate amounts of jitter into the transmitted signal, and while maintaining a roughly constant extinction ratio.
One conventional mechanism for controlling the extinction ratio is to introduce a tone modulated onto the high current limit. This may be seen from FIG. 5, in which a varying current tone is modulated around the high current needed to generate the high optical level. This tone modulation is represented in FIG. 5 by the smaller bi-directional arrow 501 traveling along the optical power versus laser current curve. For clarity, only the optical power and laser current curve for one temperature (i.e., 25 degrees Celsius) is shown in FIG. 5.
The current modulation on the high current IHIGH25 as represented by bi-directional arrow 502 thus causes a corresponding optical power modulation on the high power level PHIGH25 as represented by bi-directional arrow 503. Given a relatively constant current modulation 502, the magnitude of the optical high power level modulation 503 is a function of the slope of the laser current versus optical power curve in the linear region. From this information, the magnitude of the current change IMOD needed to transition from a high to a low optical power level may be calculated.
The frequency spectrum of the current modulation 502 (and thus also the optical modulation 503) is generally a single tone, or as close to a single tone as the hardware is capable of generating. However, the frequency spectrum of the data represented by the optical signal or other ambient noise can interfere with the single tone thereby making it difficult to detect the optical tone magnitude, and thereby could disrupt the ability to adjust the modulation current IMOD to the appropriate levels. Furthermore, the single tone frequency may interfere with the proper functioning of surrounding circuitry that is sensitive to the frequency of the tone.
Accordingly, what would be advantageous is a mechanism for controlling the extinction ratio with less susceptibility to ambient noise and the surrounding data frequency characteristics, and that is less interfering with surrounding circuitry.